This research will integrate developmental psychophysiology and psychopathology by investigating error-related brain activity in pediatric obsessive-compulsive disorder (OCD) and trichotillomania (TTM) both before and after cognitive-behavioral intervention. The error-related negativity (ERN) is an event-related brain potential (ERP) that has been associated with response monitoring and error detection. The ERN has been source-localized to the anterior cingulate cortex (ACC), which is part of the hyperactive fronto-striatal-thamalic circuitry in OCD (Stein, 2000). Thus, an enhanced ERN may serve as a marker of ACC hyperactivity relevant to the pathophysiology of OCD. Previous research has found ERN magnitude to be significantly correlated with symptom severity in a group of adult patients with OCD (Gerhing et al., 2000). The current research extends previous studies to pediatric populations, and includes pediatric TTM and pediatric controls. There is controversy about whether or not TTM is part of the OCD spectrum disorders, and the current study will provide psychophysiological data that may help differentiate these disorders. It is hypothesized that pediatric OCD patients will show enhanced error-related brain activity at protreatment, relative to both pediatric TTM patients and pediatric controls. Pediatric TTM patients are not expected to differ from pediatric controls with respect to error-related brain activity, supporting the notion that TTM is not simply a variant of OCD. Finally, the current study will also investigate the ERN after pediatric patients with OCD and TTM complete a course of CBT. It is hypothesized that only the error-related brain activity in pediatric OCD will change over the course of CBT reflecting the post-treatment alteration in the fronto-striatal-thalamic circuitry that underlies OCD.